A RECIPROCATING HERMETIC COMPRESSOR

Abstract: A reciprocating hermetic compressor, of the type comprising a cylinder block, in which there is defined a cylinder (21), lodging a reciprocating piston (31), operatively mounted to a crankshaft eccentric portion (11) of a crankshaft (6), further comprising a second cylinder (22), lodging a second reciprocating piston (32), which is operatively mounted to said crankshaft eccentric portion (11) and disposed at 90 DEG from the first cylinder (21); a counterweight means (13), incorporated to the crankshaft (6), so that its inertial force presents components, which are projected on a first axis X and on a second axis Y, containing, respectively the geometric centers of the first reciprocating piston (31) and crankshaft (6) and of the second reciprocating piston (32) and crankshaft (6), in order to balance the inertial forces of said reciprocating pistons (22, 32) and crankshaft eccentric portion (11).

Description: A RECIPROCATING HERMETIC COMPRESSOR Field of the Invention The present invention relates to a reciprocating hermetic compressor, used in small refrigeration appliances, and particularly to a construction for the piston-crankshaft assembly of said compressor. Background of the Invention The known reciprocating compressors comprise a connecting rod, whose smaller eye is articulated through a wrist pin to a piston, which reciprocates inside the cylinder of the compressor and whose larger eye is mounted to an eccentric end of a crankshaft, which is orthogonal to the piston stroke and whose other end supports the rotor of an electric motor, the stator of the motor being supported by a cylinder block lodging the bearing of the crankshaft, the cylinder block being mounted to a housing through springs, said housing being covered by a lid, defining a sealed unit. To obtain a better performance of the compressor, it is important that, in mechanical terms, the parts with relative movement are perfectly balanced. It is known that the inertia and rotation forces produced by a mechanism with a single piston cause unbalance in hermetic compressors. The maximum of the inertial force in a piston occurs at the regions of the upper and lower dead points. Said force has two important components in frequency, one main or fundamental frequency in 60 Hz and a secondary one in 120 Hz, both causing vibration in the motor-compressor assembly and, consequently, in the housing where said assembly is mounted. The 60Hz frequency is the approximate frequency in which the compressor operates. These vibrations result from the non-balancing of the forces in the gas pumping unit of the compressor, more precisely in the piston-connecting rod-shaft eccentric end assembly during operation. In these compressors, the 60Hz component is not balanced. It is, at maximum, reduced by the provision of counterweights at the crankshaft or at the rotor of the motor of said compressors, in order to balance the levels of longitudinal and transversal vibrations of the housing. These counterweights, however, rotate together with the crankshaft and produce rotary forces in the system (figure 3) that are not balanced. In order that the vibration in the housing reaches acceptable levels, the motor-compressor assembly is mounted inside said housing through springs (figure 1). Nevertheless, this mounting solution through springs has some inconveniences. During transportation, at the periods of the compressor stops and starts, or even occasionally during operation, the motor-compressor assembly is submitted to forces that make said assembly move inside the housing. To avoid impacts between said assembly and the housing during such movement, the dimensions of said housing are increased. Another problem refers, principally, to the transmission of vibrations from the motor-compressor assembly to the housing, through the discharge and suction tubes, which connect, respectively, the discharge muffler and suction muffler of these compressors to the housing. To minimize the transmission of vibration, as well as to avoid the problems of material fatigue, said tubes should be sufficiently flexible. Such flexibility is obtained by using a long discharge tube. Nevertheless, this solution causes a superheating of the suction gas, due to the heat transfer into the internal medium of the housing through said discharge tube. Plastic suction chambers and direct or semidirect connections are used to reduce the superheating, but with cost increase. Another solution to minimize the effects of vibration in the compressor resulting from the 60 Hz component discussed above would be the use of two pistons, disposed one from the other by 1800, each piston being mounted to a respective eccentric portion of the crankshaft. However, the construction of said crankshaft with two eccentric portions would be complicated. Disclosure of the Invention Thus, it is the object of the present invention to provide a reciprocating hermetic compressor, whose construction eliminates at least the 60 Hz component of the vibration forces, without producing the inconvenient effects cited above and whose construction and assembly are easy to be carried out. This and other objectives are attained in a reciprocating compressor, of the type comprising a hermetic housing; a cylinder block mounted inside the housing and defining a cylinder; a piston, which reciprocates inside the cylinder and which is operatively mounted to an eccentric portion of a crankshaft, said compressor further having a second cylinder, attached to the cylinder block and lodging a second reciprocating piston, which is operatively mounted to said crankshaft eccentric portion, said second cylinder being disposed at 900 from the first cylinder; a counterweight means, incorporated to the rotating mass of the crankshaft, so that its inertial force presents a component, which is projected on a first axis X, parallel to the geometric axis of the first reciprocating piston, in order to balance the inertial forces relative to said first piston and the component of inertial force of the crankshaft eccentric portion projected on said first axis X, and a component projected on a second axis Y, parallel to the geometric axis of the second reciprocating piston, in order to balance the inertial forces relative to said second piston and the corresponding component of inertial force of the crankshaft eccentric portion projected on said second axis Y. Brief Description of the Drawings The invention will be described below, with reference to the attached drawings, in which: Fig. 1 shows schematically a longitudinal sectional view of a reciprocating hermetic compressor, illustrating a piston-crankshaft assembly of the prior art; Fig. 2 shows, schematically and partially, the forces acting on a piston, reciprocating inside the cylinder and mounted to a crankshaft eccentric portion, said piston being at the upper dead point, according to the prior art; Fig. 3 illustrates the reciprocating piston of fig. 2, at a position intermediate to the upper and lower dead point positions, according to the prior art; and Fig. 4 shows, schematically and partially, the forces acting on a piston-crankshaft eccentric portion assembly, according to the present invention. Best Mode for Carrying Out the Invention According to figure 1, a motor-compressor assembly is suspended inside a hermetically sealed housing 1, through springs 2. A cylinder block 3 of said motorcompressor assembly serves as a support for a stator 4 of the electric motor and presents a bearing 5, for supporting a crankshaft 6 which, in this construction, carries at the lower part thereof, the rotor 7 of said motor. Said cylinder block further lodges a cylinder 8, in which inside reciprocates a piston 9, driven by the rotation of the crankshaft 6, which is mounted by means of a connecting rod 10 to a crankshaft eccentric portion 11, which is provided at an upper end portion 6a of said crankshaft 6 and which has a free upper end 12. In order that during operation there occurs balance between the levels of longitudinal and transversal vibrations, resulting from the presence of rotational and inertial forces in the motor-compressor assembly, in this prior art construction there are provided counterweight regions 13 at the crankshaft-rotor assembly, said regions being generally indicated as a region defined at the upper portion 6a of said crankshaft 6, radially opposite to the region where the crankshaft eccentric portion 11 is disposed, and as a region located close to the rotor 7, such as illustrated in figure 1, for example. The counterweight 13 is calculated, in such a way that, in the upper and lower dead point conditions, the force produced by said counterweight 13 acts in a system of forces including the piston 9 and the crankshaft eccentric portion 11, in order to balance the inertial forces relative to said elements. When the reciprocating piston 9 is at the upper dead point position (fig. 2), the inertial forces of the movement acting on the piston-crankshaft eccentric portion assembly are balanced, meaning that a centrifugal force FCw of the counterweight 13 counterbalances the summing of the centrifugal force Fe of the crankshaft eccentric portion and of the inertial force F p of the piston, i.e., Fcw = Fe +Fp When not in the upper and lower dead point conditions, the system of forces is unbalanced (fig. 3), since the force FCw, due to the counterweight 13, is superior to the summing of the forces Fe, due to the crankshaft eccentric portion, and Fp, due to the piston. This unbalance results from the fact that the inertial force Fp relative to the piston varies during the operational cycle of the latter. The situations of higher unbalance occur between the upper and lower dead point conditions, more precisely when the reciprocating piston is at the positions corresponding to 1/4 and 3/4 of the cycle. In these positions, the force relative to the piston is null, but the force relative to the crankshaft eccentric portion has an intensity equivalent to the sum of the forces associated to both the crankshaft eccentric portion and piston when aligned. Such conditions of unbalance between the forces allow the occurrence of vibrations in the system, as discussed above. The hermetic compressor of the present invention, illustrated in figure 4, has a cylinder block 20, lodging a first cylinder 21 and a second cylinder 22, each of said cylinders respectively lodging first and second reciprocating pistons 31, 32, both driven by the crankshaft eccentric portion 11 mounted to the crankshaft 6. According to the present invention, said cylinders are disposed one from the other by 900, each piston being driven by the crankshaft eccentric portion 11 through a respective connecting rod 23, 24. The force balance condition is achieved in the present invention by using reciprocating piston-connecting rod assemblies having identical inertial effects, obtained, in the preferred illustrated construction, with identical reciprocating piston-connecting rod assemblies. In a preferred form of carrying out the invention, only the assembly consisting of the first cylinder 21 and first reciprocating piston 31 acts on the gas pumping of the compressor, whereas the assembly consisting of the second cylinder 22 and second reciprocating piston 32 acts sol-ely on the balance of the forces acting on the first cylinder 21-first reciprocating piston 31 assembly, during the compressor operation, more particularly to eliminate the components of the 60 Hz forces discussed above. Nevertheless, said assembly consisting of the second cylinder 22 and second piston 32 may be formed in such a way as to be able to receive a larger eye from each of said connecting rods 23, 24. With this construction, when the first reciprocating piston 21 is at the upper dead point condition, the second reciprocating piston 22 will be at one of the 1/4 and 3/4 positions of the piston cycle, said positions being a relative function between the respective cylinders. In a preferred and illustrated embodiment, the elimination of the 60 Hz component from the inertial forces acting on the piston-crankshaft eccentric portion assembly occurs with the provision of one or more couterweights in the crankshaft-rotor assembly, diametrically to the crankshaft eccentric portion 11 of said crankshaft, so that the force resultant, relative to said couterweights, balances the inertial forces acting on the pistons and eccentric portions, during each operative cycle of said piston, as described ahead. When the first reciprocating piston 31 is at the upper dead point condition, the inertial forces acting upon the piston-crankshaft eccentric portion assembly are balanced, i.e., the summing of the inertial force Fpl of the first piston 31, with the component of the inertial force Fex of the crankshaft eccentric portion projected on an axis X, parallel to the geometric axis of the first cylinder 21 is equivalent to the component of the inertial force Fcwx of the counterweight 33 on said axis X, whereas the summing of the inertial force Fp2 of the second reciprocating piston 32 with the component of the inertial force F cwy of the crankshaft eccentric portion, Feyl projected on an axis Y, parallel to the geometric axis of the second cylinder 22, is equivalent to the component of the centrifugal force of the counterweight 33 onto said axis Y. This balance means that, mathematically, the following equations should be simultaneously obeyed: Fpl = Fex = F cwx and Fp2 = Fy = F cwy In order that this condition of balance be maintained in any phase of the gas pumping operative cycle of the compressor, the only thing to do is to determine through the equations above described the intensity of the force relative to the counterweight 33, when any one of the pistons from the piston-crankshaft eccentric portion assembly is at one of the upper or lower dead point conditions. Since the pistons are equal and the respective cycles of movements thereof are displaced at 900, a reduction in the intensity of the force relative to one of said pistons is compensated by a corresponding increase of intensity at the other of said pistons and vice-versa. The resultant of the force relative to the counterweight is therefore a well determined constant. With this construction, the more intense component of the vibration forces acting upon the motor-compressor assembly, which is the 60 Hz component, is eliminated, allowing said motor-compressor assembly to be fixedly mounted to the housing, dispensing the suspension springs of the prior art. This construction enables the use of a housing of smaller dimensions, besides the use of a shorter discharge tube and a simpler and direct suction tube, said alterations reducing the superheating of the suction gas, as well as costs.

Claims: CLAIMS 1. A reciprocating hermetic compressor, of the type comprising a hermetic housing; a cylinder block mounted inside the housing and defining a cylinder (21); a piston (31), which reciprocates inside the cylinder (21) and which is operatively mounted to a crankshaft eccentric portion (11) of a crankshaft (6), characterized in that it further comprises a second cylinder (22), attached to the cylinder block and lodging a second reciprocating piston (32), which is operatively mounted to said crankshaft eccentric portion (11), said second cylinder (22) being disposed at 900 from the first cylinder (21); a counterweight means (13), incorporated to the rotating mass of the crankshaft (6), so that its inertial force presents a component, which is projected on a first axis X, parallel to the geometric axis of the first reciprocating piston (31), in order to balance the inertial forces relative to said first piston (31) and the component of inertial force of the crankshaft eccentric portion projected on said first axis X, and a component projected on a second axis Y, parallel to the geometric axis of the second reciprocating piston (32), in order to balance the inertial forces relative to said second piston (32) and the corresponding component of inertial force of the crankshaft eccentric portion (11) projected on said second axis Y. 2. Compressor, as in claim 1, characterized in that each reciprocating piston (22, 32) is operatively mounted to the crankshaft eccentric portion (11) through a respective connecting rod. 3. Compressor, as in claim 2, characterized in that the second cylinder (31) is incorpdrated to the cylinder block. 4. Compressor, as in claim 3, characterized in that the counterweight means (13) comprises a counterweight, diametrically opposite to the crankshaft eccentric portion (11) at the rotating mass of the crankshaft (6) 5. Compressor, as in claim 4, characterized in that the reciprocting piston-connecting rod assemblies (22, 32) have identical inertial effects.

Inventors: LILIE DIETMAR ERICH BERNHARD, LILIE, DIETMAR, ERICH, BERNHARD
Applicants: BRASIL COMPRESSORES SA, LILIE DIETMAR ERICH BERNHARD, EMPRESA BRASILEIRA DE COMPRESSORES S/A. - EMBRACO, LILIE, DIETMAR, ERICH, BERNHARD
Application year: 1994
Main classification: F04B35
IPC codes: F04B35, F04B39